1. Field
This disclosure relates to waveguide devices used to combine or separate two orthogonal modes, also known as ortho-mode transducers (OMTs).
2. Description of the Related Art
Satellite broadcasting and communications systems may use a first signal having a first polarization state for the uplink to the satellite and a second signal having a second polarization state, orthogonal to the first polarization state, for the downlink from the satellite. Note that two circularly polarized signals are orthogonal if the e-field vectors rotate in the opposite directions. The polarization directions for the uplink and downlink signals may be determined by the antenna and feed network on the satellite.
A common form of antenna for transmitting and receiving signals from satellites consists of a parabolic dish reflector and a feed network where orthogonally polarized modes travel in a common waveguide. The common waveguide may typically be cylindrical or square, but may be elliptical or rectangular. In this patent, the term “cylindrical waveguide” means a waveguide segment shaped as a right circular cylinder, which is to say the cross-sectional shape of the waveguide segment is circular. Similarly, the terms “elliptical waveguide”, “rectangular waveguide”, and “square waveguide” mean a waveguide segment having an elliptical, rectangular, or square cross-sectional shape, respectively. An ortho-mode transducer may be used to launch or extract the orthogonal linearly polarized modes into or from the cylindrical waveguide.
An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. Within this description, the term “port” refers generally to an interface between devices or between a device and free space. A port of a waveguide device may be formed by an aperture in an interfacial surface to allow microwave radiation to enter or exit a waveguide within the device.
The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this document, the terms “support” and “supporting” mean that a waveguide will allow propagation of a mode with little or no loss. In a feed system for a satellite antenna, the common waveguide may be a cylindrical waveguide. The two orthogonal linearly polarized modes may be TE11 modes which have an electric field component orthogonal to the axis of the common waveguide. When the cylindrical waveguide is partially filled with a dielectric material, the two orthogonal linearly polarized modes may be hybrid HE11 modes which have at least some electric field component along the propagation axis. Two precisely orthogonal TE11 or HE11 modes do not interact or cross-couple, and can therefore be used to communicate different information.
The common waveguide terminates at a common port, which is to say that a common port aperture is defined by the intersection of the common waveguide and an exterior surface of the OMT.
Each of the two branching waveguides of an OMT typically supports only a single linearly polarized TE10 mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this document, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures.
A traditional OMT, for example as shown in U.S. Pat. No. 6,087,908, has one branch waveguide axially aligned with the common waveguide, and one branch waveguide normal to the common waveguide. The branch waveguide that is axially aligned with the common waveguide terminates at what is commonly called the vertical port. The linearly polarized mode supported by the vertical port is commonly called the vertical mode. The branch waveguide which is normal to the common waveguide is terminated at what is commonly called the horizontal port. The branch waveguide that terminates at the horizontal port also supports only a single polarized mode commonly called the horizontal mode.
The terms “horizontal” and “vertical” will be used in this document to denote the two orthogonal modes and the waveguides and ports supporting those modes. Note, however, that these terms do not connote any particular orientation of the modes or waveguides with respect to the physical horizontal and vertical directions.
Elements in the drawings are assigned reference numbers which remain constant among the figures. An element not described in conjunction with a figure may be presumed to be the same as a previously-described element having the same reference number.